Method of making single crystal cores of a ferrite including cobalt and cores so made



United States Patent METHOD OF MAKING SINGLE CRYSTAL CORES OF A FERRITEINCLUDING COBALT AND CORES S0 MADE Richard M. Bozorth, Short Hills,N.J., assignor to Bell Telephone Laboratories, Incorporated, New York,N.Y., a corporation of New York Application October 1, 1954, Serial No.459,730

2 Claims. (Cl. 336-218) The present invention relates to methods ofproducing magnetic cores for inductance coils, which cores have eithervertically elongated, rectangular, hysteresis loops or flat, lenticular,small-area, hysteresis loops and to inductance coils employing saidcores. More particularly, it relates to methods of cutting inductancecoil cores from single crystals of those ferrites which include acomponent of cobalt of from one to thirty percent by .Weight, and

treating said cores to alter their hysteresis loops in either of theabove mentioned manners and to inductance coils having said treatedcores.

In general, any ferrite including from one to thirty percent by weightof cobalt will exhibit the characteristics described hereinbelowirrespective of whether other nonferrous metallic components are presentin the particular ferrite or not. For example, cobalt zinc ferrite.conforming to the formula Co Zn Fe Q exhibits the characsenting thehysteresis loop of a magnetic material the values of the magnetic fieldH in oersteds are displayed horizontally and the resulting values of theflux density B in the material are displayed vertically. Thisconventional presentation is assumed for the purpose of descriptionthroughout this application and in the appended claims.

Magnetic materials having vertically elongated substan tiallyrectangularhysteresis loops are, as is well known to those skilled in the art, ofsubstantial value for use in switching arrangements and as informationstorage devices. Conversely, magnetic materials having a small-area,substantially flat, hysteresis loop are of substantial value ininstances where a core material of low hysteresis loss and virtuallyconstant permeability over a wide range of magnetic field strength isdesired as, for example, for loading coil cores and the like, where avirtually constant inductance over a wide range of magnetic fieldstrengths is desired.

The principal object of the invention is to provide a new and convenientmethod of producing magnetic core elements having either verticallyelongated, rectangular, hysteresis loops or small-area, lenticularhysteresis loops.

Another object is to provide inductance coil cores of a ferriteincluding cobalt, said cores having vertically elongated, substantiallyrectangular, hysteresis loops.

A further object is to provide core elements of a ferrite includingcobalt, said core elements being suitable for fabricating into elementshaving desirable properties for use as information storage devices andin switching arrangements.

A still further object is to provide inductance coil cores of a ferriteincluding cobalt, said cores having small-area, lenticular, hysteresisloops.

Other and further objects will become apparent during the course of thefollowing detailed description illustrating applications of the generalmethods of the invention, and from the appended claims.

In the accompanying drawings:

Fig. 1 shows, by way of specific example, a hollow square core of cobaltzinc ferrite which can be employed in demonstrating the modification ofthe magnetic properties of the material in accordance with theprinciples of the present invention; and

Fig. 2 shows, by way of illustration, the hysteresis loop of the core ofFig. 1 before treatment and a plurality of modifications of thehysteresis loop of the core of Fig. 1 after being subjected to a likeplurality of differing treatments, in accordance with the principles ofthe present invention, respectively.

In general ferrites of the class designated by the above defined phrasea ferrite including cobalt (i.e., any and all ferrites having from oneto thirty percent of cobalt by weight, irrespective of Whether othernon-ferrous metallic components are present or not), have a cubiccrystalline structure representable by the three mutually orthogonallyrelated crystal axes a, b, and c, the unit length along each axis beingthe edge of the unit cube.

Cores of the present invention, as illustrated by the specific example10 shown in Fig. l, are cut from a single crystal of the particularferrite including cobalt and may be cut with the front and rear surfacesparallel to any one of the three planes defined as including any two ofthe above mentioned three crystal axes a, b, and c. The cut must,furthermore, be made so that one of the two pairs of parallel legs ofthe square core 10 is parallel to one crystal axis and the other pair ofparallel legs is parallel to the other crystal axis, of the two axesselected to determine the plane with which the front and rear surfacesof the core 10 of Fig. l are to be parallel.

The particular core 10 of Fig. 1 has front and rear faces 0146 squareWith a central opening 0.062" square and a thickness front to rear of0.030. The crosssectional shape of the legs need not be rectangular butcan be round or ovoid, if desired, so long as the orientations withrespect to the axes of the crystal from which the core is cut are asdescribed above.

It should be particularly noted also that the core need not necessarilybe square but can alternatively be of any rectangular shape which can beconveniently cut from a single crystal with the orientations givenabove.

Windings, such as 12 and 14, of insulated conductive wire are placed onone or more of the legs of the core 10 and connected in series aiding toterminals 16 and 18.

Curve 20, 22 of Fig. 2 shows the hysteresis loop of the core 10 of Fig.1 before being subjected to any treatment in accordance with theprinciples of the present invention.-

If the core 10 of Fig. l is subjected to a unidirectional magnetic fieldof from two to twenty oersteds or more by passing a suitable continuouscurrent through its windings 12 and 14 and heated to a temperature offrom to degrees centigrade or more and maintained under said magneticfield and at said temperature for from a few seconds to an hour or moreand thereafter cooled to room temperature while still maintaining saidfield, a pronounced squaring of the corners of its hysteresis loop, aswell as a vertical elongation of the loop, will be found to have beeneffected, as illustrated, by way of example, by curves 24, 26; 28, 30and 32, 34

' of Fig. 2, the specific magnetic field strength, tempera- 3 ture andtime of treatment being as given below for these particular curves,respectively. Cobalt zinc ferrite of the formula given hereinabove wasemployed in these tests.

There is no advantage to using a temperature greater than the Curietemperature of the ferrite being treated, since above the Curietemperature, magnetic substances become non-magnetic. For the class offerrites (as defined above) of interest in connection with the presentinvention the Curie temperature is in the range of 300 to 500 degreescentigrade. The cobalt zinc ferrite mentioned hereinabove has, by way ofspecific example, a Curie temperature of 450 degrees centigrade.

In general the nearer the temperature approaches the Curie temperaturethe shorter the time required for treatment in accordance with thepresent invention, a limit in the order of several seconds beingsulficient at temperatures closely approaching the Curie temperature. Atlower temperatures a longer time is required, about fifteen minutesbeing adequate at 400 degrees centigrade and proportionately longertimes being advisable at still lower temperatures.

In other tests, comparable hysteresis loops to those of the three curvesjust described were obtained at magnetic annealing temperatures as lowas 140 degrees centigrade. In general, magnetic fields higher thantwenty oersteds and/or temperatures higher than 400 degrees centigrade,do not produce any perceptible further elongation of the loop orsquaring of the corners of the rectangular hysteresis loop. i.e., theoriginal hysteresis loop curve 20, 22 of Fig. 2 is obtained again if thetreated core is heated to 400 degrees centigrade in the absence of anyappreciable magnetic field and then cooled to room temperature.

Conversely, when a magnetic field is applied perpendicularly to thefront and rear faces of the core 10 by external magnetizing means, whilethe core 10 is heated to a temperature in the order of from 120 to 400degrees centigrade, or more, the field and temperature being The processis completely reversible,

maintained for a time interval depending upon the. temto roomtemperature restores the element to a normal hysteresis loop for thematerial as illustrated by curve 20, 22 of Fig. 2. This, of course,demonstrates that the last stated process is also reversible.

Similar metallic ferrites containing no cobalt, such as, for example,manganese zinc ferrite, exhibited no corresponding changes in thecharacter of their respective hysteresis loops when subjected to theabove described processes using fields up to twenty oersteds, or more,at temperatures in the range between degrees centigrade and 400 degreescentigrade, or more.

Numerous and varied other related methods, applications and arrangementsin accordance with the principles of the present invention can bereadily devised by those skilled in the art without departing from thespirit and scope of said principles.

What is claimed is:

1. The method of producing a magnetic core which comprises cutting ahollow rectangular core element from a single crystal of the ferrite COZn Fe Q the front and rear surfaces of the element being parallel to aplane defined by two crystallographic axes of the single crystal, eachof the longitudinal axes of the four portions of the hollow rectangularelement being parallel to one of the two axes of the crystal,magnetizing the element with a unidirectional magnetic field inducedwithin the element as a closed magnetic path, heating the element to atemperature between 120 degrees centigrade and the Curie temperature ofthe ferrite, maintaining the magnetization and the temperaturesimultaneously for a predetermined time interval, and cooling theelement to room temperature While maintaining the magnetizing field.

2. A hollow rectangular magnetic core element of the ferrite Co Zn Fe Ocut from a single crystal and having the front and rear surfaces of thecore parallel to a plane defined by two crystallographic axes of thesingle crystal, each of the longitudinal axes of the four portions ofthe hollow rectangular element being parallel to one of the two axes ofthe crystal, said core having been magnetically annealed to have asubstantially rectangular hysteresis loop.

References Cited in the file of this patent UNITED STATES PATENTS2,165,027 Bitter July 4, 1939 2,569,468 Gaugler Oct. 2, 1951 2,692,978Galt Oct. 26, 1954 FOREIGN PATENTS 143,757 Australia Oct. 10, 1951143,668 Australia Oct. 4, 1951 OTHER REFERENCES Magazine Article:General Electric Review, Orient ed Crystals. August 1950.

2. A HOLLOW RECTANGULAR MAGNETIC CORE ELEMENT OF THE FERRITECO0.32ZN0.22FE2.2O4 CUT FROM A SINGLE CRYSTAL AND HAVING THE FRONT ANDREAR SURFACES OF THE CORE PARALLEL TO A PLANE DEFINED BY TWOCRYSTALLOGRAPHIC AXES OF THE SINGLE CRYSTAL, EACH OF THE LONGITUDINALAXES OF THE FOUR PORTIONS OF THE HOLLOW RECTANGULAR ELEMENT BEINGPARALLEL TO ONE OF THE TWO AXES OF THE CRYSTAL, SAID CORE HAVING BEENMAGNETICALLY ANNELED TO HAVE ASUBSTANTIALLY RETANGULAR HYSTERESIS LOOP.